1. Field of the Invention
The invention relates to a coolable casing of a gas turbine or the like.
2. Description of the Related Art
EP 0 516 322 B1, on which the invention is based, discloses a coolable casing for a gas turbine. The casing is formed by a plurality of arcuate casing segments which, contiguous with one another in a circumferential direction, form a casing ring surrounding a rotor of a high-pressure turbine stage. For cooling the side of the casing segments which faces away from the rotor, an annular casing cooling chamber is provided, which extends in the radial direction between the casing segments and arcuate guide segments. The guide segments are formed from sheet metal portions which are provided with a multiplicity of passage orifices. There is also an air guide chamber which extends in the radial direction between the guide segments and a housing-side carrier or carrier segment. The carrier has, furthermore, an air supply duct which opens into the air guide chamber.
In order to cool the casing, cooling air is fed into the air supply duct. It passes from there through the passage orifices, high-velocity air jets being formed which impact essentially perpendicularly on the rear side of the casing segments. After impact, they are deflected, and a transverse flow is established in the casing cooling chamber.
The high cooling effect capable of being achieved by means of this device is based, in particular, on the combination of impact cooling and convection cooling. In order to utilize optimally the particularly advantageous heat transmission of the impact cooling which occurs, it is particularly important to achieve as high a velocity as possible of the cooling-air jets emerging through the passage orifices. A basic precondition for this is the establishment of as high a pressure difference as possible between the air guide chamber and the casing cooling chamber.
The leakage loss occurring due to flows around the sides of the guide segments presents a problem in this respect. In order to avoid such leakages, the individual guide segments are therefore soldered to the carrier completely and continuously. The outlay necessary for this purpose is enormous and therefore results in high production costs. Furthermore, this design also presents problems because casing segments of this kind are at great risk of being damaged, particularly where modern gas turbines with extremely high turbine inlet temperatures are concerned. If an exchange or a repair of the guide segments becomes necessary, overproportionally high costs are incurred, these being attributable, inter alia, to the associated soldering work.
This type of connection of the guide segments to the carrier also presents problems with regard to transient operations, such as, for example, during the run-up of the gas turbine or in the event of load changes, since, in these operating states, high temperature gradients may occur within the structural parts and subassemblies and may lead to high mechanical stresses. The soldered joint between the guide segments and the carrier is of particular risk under these circumstances.
The invention attempts to avoid the disadvantages described. The object on which it is based is to specify a coolable casing of the type initially mentioned, which is designed in a simple way without any serious losses of cooling efficiency, with the result that both the production costs and the repair and maintenance costs can be reduced. Furthermore, the mechanical loads occurring during the transient operations described are to be reduced and an increased useful life is thus to be achieved.
This is achieved, according to the invention, in that, in a coolable casing, the guide segments are mounted loosely and with radial play. This type of mounting makes it possible to have relative movements between the carrier or carrier segment and the casing segments. The radial play is dimensioned in such a way that an essentially unimpeded relative movement is possible, even for the most unfavorable transient operating state. The latter occurs during the run-up phase, in which the guide segments are subjected to cooling air which is at a comparatively high temperature, with the result that, by contrast, the carrier is still comparatively cold.
A particularly simple design can be implemented when the guide segments are guided loosely between the carrier and spacers, the spacers being attached to the rear side of the casing segments so as to project in the radial direction. The cooling airstream impinging onto the guide segments presses these against the spacers, thereby maintaining a permanently predetermined spacing between the guide segments and the rear side of the casing segments. The casing cooling chamber is consequently fixed in the radial direction, the radial extent of the latter corresponding to the height of the spacers. The comparatively high pressure under which the cooling air is supplied ensures that, during the time when the guide segments are subjected to cooling air, they are held pressed reliably against the spacers.
Ribs allowing the guide segments to be supported continuously along a continuous line have proved particularly appropriate as spacers. Punctiform supporting elements are also suitable, such as, for example, pins or elevations of cylindrical or conical design, which are arranged, in principle, in any desired way and thereby allow an even better equalization of the cooling effect.
A particularly reliable mounting of the guide segments can be achieved when these are provided with at least two radial webs which engage with slight axial play into corresponding guide grooves of the carrier. The slight play, on the one hand, allows the radial displacement of the guide segments and, on the other hand, minimizes the leakage losses due to the flow around the sides of the guide segments, even when the cooling air is supplied at a comparatively high excess pressure.
It is particularly beneficial to design the guide segments with a U-shaped cross-sectional profile which can be produced in a particularly simple way. By means of a noncutting forming operation, in each case legs can be formed laterally which, as webs which are continuous in a circumferential direction, ensure that the respective guide segment is guided accurately.
Preferably, the guide segments are arranged so as to overlap in a circumferential direction. This gives rise, in a circumferential direction, to a continuous uninterrupted parting plane between the casing cooling chamber and the air supply duct, so that leakage losses at the transitional points of two guide segments arranged in each case next to one another are further minimized.
An increased number of passage bores may be provided in the overlap region, in order to make the formation of cooling-air jets in sufficient quantity available even in this region. This takes into account the effect that, due to the loose mounting of the individual guide segments, relative assignment may vary in a circumferential direction, together with the risk that, in the overlap region, too few passage bores of two overlapping guide segments come into congruence.
It is, of course, also possible, instead of an increased number of passage bores, to provide in the overlap region, in each case in one of the two guide segments, passage orifices with an enlarged cross section in a circumferential direction, so that the passage bores remain free, irrespective of the relative position of two adjacent guide segments which is momentarily assumed.
Flange portions running in the circumferential direction are provided in the contact region in each case between the casing segment and carrier, so that the casing segment and carrier are releasably connected to one another by means of holding clamps which engage round the flange portions in each case contiguous with one another. The holding clamps, on the one hand, press the casing segments and carrier firmly against one another, so that leakage losses due to cooling air emerging between the two structural parts is largely prevented. On the other hand, the holding clamps make it possible to release and restore the connection in a simple way, so that not only the mounting of the casing, but, to a particular extent, also repair are greatly simplified by the exchange of individual elements.
Additional sealing elements between the holding clamps, on the one hand, and the flange portions, on the other hand, ensure virtually complete sealing off in the contact region between the casing segment and carrier. The cooling-air requirement can consequently be kept at a low level.